1. Field of the Invention
The present invention relates to a control device for controlling an image forming apparatus provided with plural loads.
2. Related Background Art
The widespread application of microcomputers in various devices in recent years has significantly contributed to the improvement in its performance, and this tendency is expected to further increase in the future. These changes rely on the advance of semiconductor technology and the cost reduction in the resulting chips. The improvement in the performance of such devices has resulted in an increase in the number of microcomputers used, and the amount of software therefor is drastically increasing year after year. In fact, the manpower required for the development of microcomputer equipment is principally occupied by the development of software, and the development of software has become a bottleneck in the period required for equipment development. This is generally referred to as the so-called "software crisis".
Semiconductor chips, being very inexpensive, are being employed in larger numbers in order to increase the added value of the equipment. Consequently the hardware is becoming simpler and improved in performance, but the software is unable to catch up to the hardware. Further, programming has to be started anew in case of a design change of the product. Though the computer sequence provides the user with a system with large flexibility, software design is extremely difficult. For this reason various equipment is now being designed on commercially available operation systems (OS). However, stand-alone office automation equipment is not large enough as to justify the use of such commercially available real-time operation systems, which are in fact inefficient in terms of the program capacity.
Despite the fact that the period for development is always too short because of the increasingly shorter life cycle of the products, the software or application program developed for any particular equipment is usually not utilized in the succeeding development. This is basically due to the conventional architecture of the microcomputers. FIG. 8 illustrates the hardware structure of a conventional control system for a mechanical-electronic equipment such as a copying machine, employing four one-chip microcomputers MC1-MC4. For this purpose, an 8-bit microcomputer NEC u-COM87AD is often used, having various I/O ports, a 4-Kbyte ROM and 128-byte RAM on a single chip, because of satisfactory cost performance.
The microcomputers MC1-MC4 control the interior of the apparatus, but each has an external serial input/output port (SI/O) for enabling comunication with external accessories through a bus-line. In the case of a copying machine, the external accessories include a sorter, collator, automatic document feeder (ADF), copy charge counter etc. which are optionally added to the copying machine. In the illustrated structure, the microcomputer MC1 is a host processor principally for sequence control of the apparatus, and controls by counting the timing pulses with a counter and comparing the number of said pulses with the address numbers of the ROM, and setting or resetting suitable actuators such as solenoids, clutches, motors etc. required for the sequence control. Also it receives signals for sequence control, from sensors and detecting switches.
The microcomputer MC2 principally controls servo motors and stepping motors. In case of a copying machine there are usually employed a few stepping motors for lens movement, sheet feeding, movement of an original support table etc., while the servo motors are employed for driving a photosensitive drum, and an optical scanner system. The microcomputer MC3 controls analog input and output signals, representing temperature, light intensity, humidity and surface potential, and also performs diagnosis of the entire equipment. These input signals, received in analog from, are converted into digital signals by A/D converters on the chip.
The microcomputer MC4 principally controls the display element, such as light-emitting diodes(LED), liquid crystal display(LCD) devices, fluorescent display tubes etc. of the display panel, and controls the signal inputs from key switches.
Such a multi-processor system is featured by a good cost performance in comparison with using one-chip microcomputers, but is associated with a drawback of complexity of protocol and communication software among the microcomputer chips. Also the communication among the microcomputers is not necessarily smooth because the serial input/output ports incorporated in these chips are often not fast enough and require a long idling time.
More specifically, in the case of data transmission to a slave microcomputer designated by the host microcomputer, all the slave microcomputers connected to the host microcomputer receive an interruption command, and suspend all the jobs in order to confirm the address of the command. A particular microcomputer, having confirmed that the command is addressed to it, starts to receive the data. Naturally said microcomputer is unable to do other jobs during such interruption procedure, and the situation is the same for the host microcomputer. Therefore, in the course, for example, of a precise servo control, the slave microcomputer refuses any data reception except for urgent signals, by sending a busy signal to the host microcomputer.
In this manner the conventional control device has required program development for each model of the equipment, and the program has to be complex in order to achieve advanced performance. Further, efficient control is difficult with plural processors due to an increased idling time. For resolving these drawbacks the present applicant has already proposed various systems as disclosed in U.S. patent application Ser. No. 795,229 filed Nov. 5, 1985, now Ser. No. 244,638 filed Sep. 12, 1988now U.S. Pat. No. 4,980,814, issued on Dec. 25, 1990; Ser. No. 836,239 filed Mar. 4, 1986 now abandoned; Ser. No. 893,915 filed Aug. 6, 1986 now U.S. Pat. No. 4,811,052, issued on Mar. 7, 1991.